1. Field of the Invention
The present invention teaches a new paradigm for a radar instrument using incoherent and coherent techniques and supporting data processing for obtaining accurate ice sounding profiles.
2. Description of the Related Art
Radar ice sounding is increasing in importance, both on earth and for planetary applications. The recent re-discovery of Lake Vostok, lying some 3 km below the surface of the Antarctic ice sheet, is one example. Not only is Lake Vostok of interest for its potential paleobiology, its observation at depth offers at least a crude approximation to the challenges of seeing many kilometers into the ice mantel of Europa, Jupiter's satellite. Substantial challenges continue to thwart efforts to survey in depth the ice sheets and glaciers of Greenland, Iceland, and Antarctica.
To date, all attempts to improve the performance of radar ice sounders are characterized by a trade-off between coherent and incoherent integration. This trade-off within the current state of the art implies a larger Signal-to-Clutter ratio (SCR) and a smaller Signal-to-Speckle Standard Deviation ratio (SSR), or a smaller SCR and a larger SSR. Up until now there has been no radar ice sounder technique that exploits the advantages of both forms of integration, nor an ice sounding radar that promotes transformation of the data into the Doppler domain, nor an ice sounder that implements waveform delay compensation and parallel processing over the data in the various Doppler bins. Some examples of conventional ice sounder techniques are given below.
Incoherent approaches to radar ice sounders include radar sounders patterned after conventional incoherent pulse-limited radar altimeters as shown in FIGS. 1(a) and 1(b), where FIG. 1(a) is an elevation view and FIG. 1(b) is a plan view of the illumination geometry of an ice sounding radar. The objective is to measure the depth h.sub.1 of the lower surface of an ice sheet beneath the upper surface, seen from an observing altitude h above the ice. (Of course, the situation generalizes to more complex layering and volume scattering.) A low loss tangent characterizes the ice, along with a velocity of propagation c.sub.1 that is typically on the order of c/1.7, where c is the speed of light in free space. Generally, ice sounding is performed from a sled or low flying aircraft. This minimizes the side scatter and maximizes the energy penetrating the surface. However, if the terrain is rough or a planetary body is being sounded, such as Europa, then the altitude h must be larger, causing a breakdown in the performance of the sounder.
An ice sounding radar has a frequency that is low for radar, typically 50-150 MHz, and a wavelength of approximately 6 m-2 m, respectively, in free space. This means that there is rather little directivity advantage to be gained from the antenna. Off-nadir scatter may arise from the surface at the same time delay as the depth signal of interest, but from nuisance features that may lie at a considerable distance from nadir.
The same geometric principles apply to an ice sounder as are encountered in pulse limited radar altimetry. The most important of these is that on any given surface (or reflecting plane at depth), concentric annuli are resolved by the intersection of the pulse and that surface. These annuli all have nominally equal areas. Thus, if there are surface scatterers illuminated by the antenna pattern that happen to be at the same radar time delay as the lower surface of the ice sheet, then their corresponding reflected signals will arrive together with the depth signal, and compete with it. These unwanted reflections are known as clutter. Since the surface reflections do not suffer attenuation from the ice, and they may arise from a large area, the resulting clutter power may be as large as or larger than that of the desired signal.
Thus, to summarize the incoherent approach to radar ice sounding, application of the conventional radar altimetry paradigm leads to results that have relatively poor SCR. Acceptable Signal-to-Noise Ratio (SNR) can be assured if sufficient transmitter power is available. This conventional approach usually leads to acceptable SSR. The incoherent approach only works well when the sensor/ice sheet geometry is such that clutter signals are not encountered. This constraint augers against larger radar altitudes, and compromises sounder effectiveness in all but the simplest ice sheet sounding opportunities. The incoherent technique is known as incoherent stacking in the geophysics community.
Another approach to the basic ice sheet sounding problem is a coherent method that uses coherent (Doppler beam-sharpened) integration as shown in FIGS. 2(a) and 2(b) which show an elevation view and a plan view, respectively. Although this approach has been shown to improve the SCR of the ice soundings, the gain against clutter is at the expense of degraded performance in other regards.
The viewing geometry is the same as before. There is one additional requirement on the radar, namely, that it must maintain pulse-to-pulse coherence in the data sequence. It follows that coherent integration in the along-track (pulse sequence) direction singles out the one Doppler window within which the phase is relatively constant. At all other Doppler locations, the higher frequency Doppler components tend to cancel each other out. Of course, this is the objective of coherent integration. The result is to reduce the effective along-track width of the sounding footprint to the diameter of the first Fresnel zone, which is centered at zero-Doppler. Coherent integration suppresses the clutter returns arising from other Doppler frequencies. This processing technique is known as unfocused SAR or Doppler beam-sharpening integration.
Clutter contributions may still arise from the across-track resolution cells within the zero-Doppler bin. These cells have areas that decrease as the square root of delay time. Hence, their net clutter contribution is very much less than occurs through incoherent processing.
The negative corollary of this simple coherent integration approach is that much of the received signal is used to cancel out itself, which has the effect of discarding potentially useful data. One consequence of this is a substantial loss in SSR, which is a major disadvantage.
Consider the SSR issue more carefully. Robust sounding waveforms require a large number of degrees of freedom, just like their altimetric counterparts. Incoherent integration of sounder pulses typically implies the summation of several hundred statistically independent waveforms for each sounding profile. On the other hand, coherent integration produces only one waveform for each integration. Of course, several of these may be combined incoherently if redundant data are available. However, the combination of simple coherent integration and its implied data discard leads to a large sacrifice in speckle suppression, a major disadvantage brought about by the means to achieve an increase in the signal-to-clutter ratio.
Thus, to summarize the coherent approach to radar sounding, application of coherent integration or Doppler beam-sharpened processing leads to results that often have low SSR. Acceptable SNR can be assured if sufficient transmitter power is available, although the coherent integration technique can compromise that parameter as well. Coherent integration usually improves SCR, and can improve SNR if the radar pulse repetition frequency is sufficiently high. The coherent approach only works well when the sensor/ice sheet geometry is such that redundant data are available to partially offset the loss of data incurred by the coherent algorithm. The coherent approach is known as coherent stacking in the geophysics community.
A remote sensing ice sounding technique that could be shown to offer improved performance simultaneously in all three principle parameters -SNR, SCR, and SSR- would open new and previously unattainable scientific possibilities.